The present invention relates to a method of manufacturing a shadow mask for a color picture tube.
A color picture tube of the shadow mask type has a shadow mask which has a plurality of apertures formed at a predetermined pitch and which is disposed close to a phosphor screen. Red, blue and green electron beams from an electron gun of the color picture tube are converged in the vicinity of each aperture and pass through this aperture. The electron beams then land on the corresponding color phosphors to reproduce a color image. When a distortion of the shape of apertures of the shadow mask, or deviations in alignment between the apertures and the corresponding phosphors, or a deviation in a predetermined distance (hereinafter referred to as a "q value") between the shadow mask and the phosphor screen exceed predetermined values, color purity is degraded (hereinafter referred to as a "purity drift"), resulting in a decisive drawback. In order to guarantee that a proportion of each electron beam is obliquely transmitted through each aperture, the aperture has a sectional shape as shown in FIG. 1. More specifically, an opening 2 (large opening) of the aperture of a shadow mask 1 at the side of the phosphor screen has an area three times that of an opening 3 (small opening) thereof at the side of the electron gun. The shadow mask of this type is manufactured as follows. As shown in FIGS. 2A and 2B, photoresist films 4 are formed on two major surfaces of a sheet 1 for forming a shadow mask. Predetermined portions of the photoresist films 4, which correspond to the prospective large and small openings 2 and 3, are exposed through the mask patterns. The exposed structure is developed to remove the portions of the films 4 corresponding to the prospective large and small openings 2 and 3, as shown in FIG. 2B, so that portions of the sheet 1 which correspond to the prospective large and small openings 2 and 3 are bared. When the sheet 1 contains iron as its major constituent, the sheet is etched, using an etching solution having ferric chloride as its major constituent, to form a predetermined aperture, as shown in FIG. 2C. Thereafter, the resist films are removed to obtain a finished flat mask. In the case of the aperture shape shown in FIG. 1, the precision of the minimum aperture size is mostly influenced by etching from the small opening side. However, etching from the large opening side must be controlled. Naturally, an etching rate from the large opening side must be greater than that from the small opening side. Since the etching area at the large opening side is about three times that at the small opening side, replacement between a new solution and a fatigued solution can be properly performed, so that the etching rate at the large opening side is greater than that at the small opening side, thereby satisfying the above condition to some extent. However, since the etching rate from the large opening side is great, side etching is also promoted. As a result, an overhang or projection 5 of the photoresist film 4 is formed to float on the sheet 1, as shown in FIG. 2C. The overhang 5 often becomes peeled off or damaged due to an impact strength of the etching solution on the sheet 1. As a result, a sheet portion from which the resist film has been damaged or peeled off, is subjected to further etching, with the result that the aperture size is distorted, thereby degrading the quality of the shadow mask. This tendency typically occurs in a high-precision micropatterned color picture tube wherein an aperture pitch and an aperture size are small, or wherein the thickness of the sheet is increased and the etching amount is increased.
In such a color picture tube, about 2/3 or more of the electron beams are bombarded against the shadow mask without being transmitted through the apertures. As a result, the shadow mask is thermally expanded, and the q value changes, so that color purity is again degraded. The dome phenomenon, resulting from thermal expansion of the shadow mask, occurs less frequently when a radius of curvature of an effective area of the shadow mask is small, since the tolerance of the electron beams can be increased. However, the radius of curvature of the effective area of the shadow mask is the same as that of the faceplate, and it is difficult for an observer to see an image on the screen when the radius of curvature is small. For this reason, the radius of curvature must be increased as far as possible. Any resultant dome phenomenon must be prevented. For this purpose, heat radiation properties of the shadow mask may be increased, or heat absorption properties of the phosphor screen may be increased to reduce the dome phenomenon. However, these methods cannot greatly decrease the dome phenomenon, and provide only a small margin of improvement. According to another method, a thickness of a shadow mask sheet is increased to decrease thermal deformation thereof. However, when the thickness of the sheet is increased, etching with high precision cannot be properly performed. For example, the etching time for a sheet having a thickness of 0.3 mm is about three times that for a sheet having a thickness of 0.15 mm. In the case of the sheet having the thickness of 0.3 mm, aperture shape distortion occurs frequently due to damage of the overhang of the resist film.